Essentially, you're intercepting the packet as it enters your network interface (eth0, in this case), but before any routing decisions have been made about it. You're then performing a translation on the packet: if the source is 212.333.111.222 and the destination port is 80, then redirect the packet to port 9020 instead.

In all other cases, traffic will flow as it normally would; ie. if the source address and destination port do not match both 212.333.111.222 and port 80, the packet will be left untouched.